%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% %A gentable.tex GAP documentation Thomas Breuer %% %A @(#)$Id: gentable.tex,v 3.9 1994/06/17 19:02:06 vfelsch Rel $ %% %Y Copyright 1990-1992, Lehrstuhl D fuer Mathematik, RWTH Aachen, Germany %% %H $Log: gentable.tex,v $ %H Revision 3.9 1994/06/17 19:02:06 vfelsch %H examples adjusted to version 3.4 %H %H Revision 3.8 1994/06/10 04:42:01 vfelsch %H updated examples %H %H Revision 3.7 1994/05/19 13:45:21 sam %H bug fixes, introduced 'PrintCharTable' %H %H Revision 3.6 1993/02/19 10:48:42 gap %H adjustments in line length and spelling %H %H Revision 3.5 1993/02/15 10:54:58 felsch %H examples fixed %H %H Revision 3.4 1993/02/13 10:05:33 felsch %H example fixed %H %H Revision 3.3 1992/04/07 23:05:55 martin %H changed the author line %H %H Revision 3.2 1992/03/27 14:04:16 sam %H removed "'" in index entries %H %H Revision 3.1 1991/12/30 08:08:05 sam %H initial revision under RCS %H %% \Chapter{Generic Character Tables}\index{character tables!generic}% \index{tables!generic}\index{library tables!generic} This chapter informs about the conception of generic character tables (see "More about Generic Character Tables"), it gives some examples of generic tables (see "Examples of Generic Character Tables"), and introduces the specialization function (see "CharTableSpecialized"). The generic tables that are actually available in the \GAP\ group collection are listed in "CharTable", see also "Contents of the Table Libraries". %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \Section{More about Generic Character Tables} Generic character tables provide a means for writing down the character tables of all groups in a (usually infinite) series of similar groups, e.g. the cyclic groups, the symmetric groups or the general linear groups ${\rm GL}(2,q)$. Let $\{G_q\|q\in I\}$, where $I$ is an index set, be such a series. The table of a member $G_q$ could be computed using a program for this series which takes $q$ as parameter, and constructs the table. It is, however, desirable to compute not only the whole table but to get a single character or just one character value without computation the table. E.g.\ both conjugacy classes and irreducible characters of the symmetric group $S_n$ are in bijection with the partitions of $n$. Thus for given $n$, it makes sense to ask for the character corresponding to a particular partition, and its value at a partition\: | gap> t:= CharTable( "Symmetric" );; gap> t.irreducibles[1][1]( 5, [ 3, 2 ], [ 2, 2, 1 ] ); 1 # a character value of $S_5$ gap> t.orders[1]( 5, [ 2, 1, 1, 1 ] ); 2 # a representative order in $S_5$| *Generic table* in \GAP\ means that such local evaluation is possible, so \GAP\ can also deal with tables that are too big to be computed as a whole. In some cases there are methods to compute the complete table of small members $G_q$ faster than local evaluation. If such an algorithm is part of the generic table, it will be used when the generic table is used to compute the whole table (see "CharTableSpecialized"). While the numbers of conjugacy classes for the series are usually not bounded, there is a fixed finite number of *types* (equivalence classes) of conjugacy classes; very often the equivalence relation is isomorphism of the centralizer of the representatives. For each type $t$ of classes and a fixed $q\in I$, a *parametrisation* of the classes in $t$ is a function that assigns to each conjugacy class of $G_q$ in $t$ a *parameter* by which it is uniquely determined. Thus the classes are indexed by pairs $(t,p_t)$ for a type $t$ and a parameter $p_t$ for that type. There has to be a fixed number of types of irreducibles characters of $G_q$, too. Like the classes, the characters of each type are parametrised. In \GAP, the parametrisations of classes and characters of the generic table is given by the record fields 'classparam' and 'charparam'; they are both lists of functions, each function representing the parametrisation of a type. In the specialized table, the field 'classparam' contains the lists of class parameters, the character parameters are stored in the field 'charparam' of the 'irredinfo' records (see "Character Table Records"). The centralizer orders, representative orders and all powermaps of the generic character table can be represented by functions in $q$, $t$ and $p_t$; in \GAP, however, they are represented by lists of functions in $q$ and a class parameter where each function represents a type of classes. The value of a powermap at a particular class is a pair consisting of type and parameter that specifies the image class. The values of the irreducible characters of $G_q$ can be represented by functions in $q$, class type and parameter, character type and parameter; in \GAP, they are represented by lists of lists of functions, each list of functions representing the characters of a type, the function (in $q$, character parameters and class parameters) representing the classes of a type in these characters. Any generic table is a record like an ordinary character table (see "Character Table Records"). There are some fields which are used for generic tables only\: 'isGenericTable':\\ always 'true' 'specializedname':\\ function that maps $q$ to the name of the table of $G_q$ 'domain':\\ function that returns 'true' if its argument is a valid $q$ for $G_q$ in the series 'wholetable':\\ function to construct the whole table, more efficient than the local evaluation for this purpose The table of $G_q$ can be constructed by specializing $q$ and evaluating the functions in the generic table (see "CharTableSpecialized" and the examples given in "Examples of Generic Character Tables"). The available generic tables are listed in "Contents of the Table Libraries" and "CharTable". %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \Section{Examples of Generic Character Tables}\index{tables!generic}% \index{character tables!generic} 1. The generic table of the cyclic group\: For the cyclic group $C_q = \langle x \rangle$ of order $q$, there is one type of classes. The class parameters are integers $k\in\{0,\ldots,q-1\}$, the class of the parameter $k$ consists of the group element $x^k$. Group order and centralizer orders are the identity function $q \mapsto q$, independent of the parameter $k$. The representative order function maps $(q,k)$ to $\frac{q}{\gcd(q,k)}$, the order of $x^k$ in $C_q$; the $p$-th powermap is the function $(q,k,p) \mapsto [1,(kp\bmod q)]$. There is one type of characters with parameters $l\in\{0,\ldots,q-1\}$; for $e_q$ a primitive complex $q$-th root of unity, the character values are $\chi_l(x^k) = e_q^{kl}$. The library file contains the following generic table\: | rec(name:="Cyclic", specializedname:=(q->ConcatenationString("C",String(q))), order:=(n->n), text:="generic character table for cyclic groups", centralizers:=[function(n,k) return n;end], classparam:=[(n->[0..n-1])], charparam:=[(n->[0..n-1])], powermap:=[function(n,k,pow) return [1,k*pow mod n];end], orders:=[function(n,k) return n/Gcd(n,k);end], irreducibles:=[[function(n,k,l) return E(n)^(k*l);end]], domain:=(n->IsInt(n) and n>0), libinfo:=rec(firstname:="Cyclic",othernames:=[]), isGenericTable:=true);| 2. The generic table of the general linear group $\rm{GL}(2,q)$\: We have four types $t_1, t_2, t_3, t_4$ of classes according to the rational canonical form of the elements\::\\ $t_1$ scalar matrices,\\ $t_2$ nonscalar diagonal matrices,\\ $t_3$ companion matrices of $(X-\rho)^2$ for elements $\rho\in F_q^{\ast}$ and\\ $t_4$ companion matrices of irreducible polynomials of degree 2 over $F_q$. The sets of class parameters of the types are in bijection with\\ $F_q^{\ast}$ for $t_1$ and $t_3$, $\{\{\rho,\tau\}; \rho, \tau\in F_q^{\ast}, \rho\not=\tau\}$ for $t_2$ and $\{\{\epsilon,\epsilon^q\}; \epsilon\in F_{q^2}\setminus F_q\}$ for $t_4$. The centralizer order functions are $q \mapsto (q^2-1)(q^2-q)$ for type $t_1$, $q \mapsto (q-1)^2$ for type $t_2$, $q \mapsto q(q-1)$ for type $t_3$ and $q \mapsto q^2-1$ for type $t_4$. The representative order function of $t_1$ maps $(q,\rho)$ to the order of $\rho$ in $F_q$, that of $t_2$ maps $(q,\{\rho,\tau\})$ to the least common multiple of the orders of $\rho$ and $\tau$. The file contains something similar to this table\: | rec(name:="GL2", specializedname:=(q->ConcatenationString("GL(2,",String(q),")")), order:= ( q -> (q^2-1)*(q^2-q) ), text:= "generic character table of GL(2,q),\ see Robert Steinberg: The Representations of Gl(3,q), Gl(4,q),\ PGL(3,q) and PGL(4,q), Canad. J. Math. 3 (1951)", classparam:= [ ( q -> [0..q-2] ), ( q -> [0..q-2] ), ( q -> Combinations( [0..q-2], 2 ) ), ( q -> Filtered( [1..q^2-2], x -> not (x mod (q+1) = 0) and (x mod (q^2-1)) < (x*q mod (q^2-1)) ))], charparam:= [ ( q -> [0..q-2] ), ( q -> [0..q-2] ), ( q -> Combinations( [0..q-2], 2 ) ), ( q -> Filtered( [1..q^2-2], x -> not (x mod (q+1) = 0) and (x mod (q^2-1)) < (x*q mod (q^2-1)) ))], centralizers := [ function( q, k ) return (q^2-1) * (q^2-q); end, function( q, k ) return q^2-q; end, function( q, k ) return (q-1)^2; end, function( q, k ) return q^2-1; end], orders:= [ function( q, k ) return (q-1)/Gcd( q-1, k ); end, ..., ..., ... ], classtext:= [ ..., ..., ..., ... ], powermap:= [ function( q, k, pow ) return [1, (k*pow) mod (q-1)]; end, ..., ..., ... ], irreducibles := [[ function( q, k, l ) return E(q-1)^(2*k*l); end, function( q, k, l ) return E(q-1)^(2*k*l); end, ..., function( q, k, l ) return E(q-1)^(k*l); end ], [ ..., ..., ..., ... ], [ ..., ..., ..., ... ], [ ..., ..., ..., ... ]], domain := ( q->IsInt(q) and q>1 and Length(Set(FactorsInt(q)))=1 ), isGenericTable := true )| %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \Section{CharTableSpecialized}\index{character tables!generic} \index{tables!generic} 'CharTableSpecialized( , )' returns a character table which is computed by evaluating the generic character table at the parameter . | gap> t:= CharTableSpecialized( CharTable( "Cyclic" ), 5 );; gap> PrintCharTable( t ); rec( identifier := "C5", name := "C5", size := 5, order := 5, centralizers := [ 5, 5, 5, 5, 5 ], orders := [ 1, 5, 5, 5, 5 ], powermap := [ ,,,, [ 1, 1, 1, 1, 1 ] ], irreducibles := [ [ 1, 1, 1, 1, 1 ], [ 1, E(5), E(5)^2, E(5)^3, E(5)^4 ], [ 1, E(5)^2, E(5)^4, E(5), E(5)^3 ], [ 1, E(5)^3, E(5), E(5)^4, E(5)^2 ], [ 1, E(5)^4, E(5)^3, E(5)^2, E(5) ] ], classparam := [ [ 1, 0 ], [ 1, 1 ], [ 1, 2 ], [ 1, 3 ], [ 1, 4 ] ], irredinfo := [ rec( charparam := [ 1, 0 ] ), rec( charparam := [ 1, 1 ] ), rec( charparam := [ 1, 2 ] ), rec( charparam := [ 1, 3 ] ), rec( charparam := [ 1, 4 ] ) ], text := "computed using generic character table for cyclic groups"\ , classes := [ 1, 1, 1, 1, 1 ], operations := CharTableOps )|